Induction heating apparatuses used to heat or melt metals operate on the principle of inducing eddy currents in the metal workpiece to be heated. The eddy currents are induced in the metal workpiece by passing an alternating current through an induction coil to generate a time-varying magnetic field, or induction field. Depending upon the magnitude and frequency of the alternating current in the induction coil, the induction field can be used for melting and/or heating the metal workpiece.
The efficiency of an induction coil to melt or heat a metal workpiece depends, in part, on the amount of energy (in the form of electromagnetic energy) which couples from the induction coil to the metal workpiece and is converted into heat energy in the metal workpiece. Present materials that are used to manufacture induction coils have the disadvantage of resistive losses within the conventional materials (i.e., copper) used to form the induction coil. In particular, anon-ferrous load of induction coils have efficiencies as low as 40% due to the current to heat them inductively is very large. The resistive losses are based on the square of the current, thus become significant when large currents are used to inductively heat a metal workpiece.
In an effort to reduce the resistive losses, some induction coils have been manufactured using superconducting materials. However, it has been found that superconductors produce losses when exposed to an alternating magnetic field. As such, the heat from the AC losses in the superconductors must be cooled at very low temperatures, which cooling can be very expensive. Superconductors have been used for some time in the medical industry for Magnetic Resonance Imaging. Superconductors have also been used in the motor industry for winding armatures to make large motors much smaller. In the area of Magnetic Hydrodynamic Drives, superconductors have been used in large ships. Transmission lines made from superconductors are used to carry large amounts of current and are in place around the United States.
One possible advance with regard to superconductors is the formation of a static or DC magnetic field that has little or no energy losses. Superconductors can, under DC conditions, conduct electric current with very little energy losses. Several types of induction coils that include superconductor materials are disclosed in U.S. Pat. Nos. 5,781,581 and 6,730,890, United States Publication No. 2006/0157476, Chinese publication No. CN 101017729, Norwegian Patent No. 308,980, and PCT Publication No. WO 03/044813, all of which are incorporated entirely herein.
Although these early uses of superconductor materials in induction coils had great potential, these superconducting materials were very expensive to use, the cooling systems requiring use of the superconducting materials was also very expensive and complicated to use, and the configuration of the induction coil that included the superconducting material was difficult to manufacture due to the configuration requirement of the superconducting material.
In view of the current state of induction coils, there remains a need for an induction coil that includes a superconducting material, and which induction coil is easier to manufacture and simpler and less costly to operate.